Life-cycle-thinking in the assessment of urban green infrastructure: systematic scoping review

Urban green infrastructure (UGI) delivers multiple social, environmental and economic benefits. A life cycle thinking (LCT) approach is proposed as a viable methodological basis for the holistic valuation of UGI. The purpose of this scoping review is to provide a thorough overview of the emerging literature on LCT application for UGI evaluation. The scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses protocol. A search was conducted in three databases to identify studies which employ LCT for UGI assessments. Eligibility screening and data extraction were performed by three reviewers. The review identified 164 relevant studies, published from 1992 to 2019 across 83 journals. There is a clear prevalence of theoretical UGI cases (54%) over real-life UGI assessments (20%). We also observed a large (84%) and increasing share of comparative studies, mostly comparing UGI with ‘grey’ infrastructure. Although cost-benefit-assessment methods initially dominated, recent studies apply LCT methods more and 7% combine both. The focus has primarily been on UGI types of smaller scale, lower biological complexity, and those integrated with built infrastructure. Green roofs are the most assessed type (40% of all cases), with urban farms second (15%). The assessments of more complex UGI objects lag behind. Most of the reviewed studies include one to three life cycle stages out of seven based on the life cycle assessment (LCA) standards, therefore very few identified studies can be considered comprehensive LCAs. Most studies assess fewer than five UGI benefits (out of 65 identified) and half assess fewer than three UGI disbenefits (out of 36 identified). The two most assessed benefits are energy savings and greenhouse gas (GHG) emissions reductions. The most assessed disbenefits are the monetary cost of implementation and maintenance and GHG emissions. Social aspects are the least assessed.


Introduction
It is widely recognised that urban green infrastructure (UGI) delivers multiple social, environmental and economic benefits contributing to urban sustainability [1][2][3][4][5][6][7][8][9][10][11][12]. The Palgrave Encyclopedia of Urban and Regional Futures, for example, identifies 41 discrete benefits across the four categories of economic, social, environmental and health and wellbeing, ranging from increase in property value and tax revenue to urban heat island mitigation, stronger community cohesion and increase in life expectancy [13]. UGI for the purposes of this study is defined as 'natural or semi-natural elements, systems or their networks within urban environment that provide ecosystem services' . In contrast, 'grey' infrastructure refers to conventional human engineered infrastructure such as sewage treatment works or asphalt paving.
What we refer to here as the benefits of UGI may also be articulated in terms of ecosystem services, defined for example by the UK National Ecosystem Assessment as 'the benefits provided by ecosystems that contribute to making human life both possible and worth living' [14]. A rich literature has developed around ecosystem services from a theoretical perspective and as a framework for decision-making both before and especially since the release of the UN's Millennium Ecosystem Assessment synthesis reports in 2005 [15]. For brevity and to be broadly inclusive we use the term 'benefits' .
Despite the existing identification of a multitude of UGI benefits, policy-and decision-makers locally, nationally and globally lack reliable data and tools for assessing and understanding the full value of UGI, which stalls uptake and limits access to funding for UGI development in cities [16]. A life cycle thinking (LCT) approach and related methods have been suggested as a viable methodological basis for assessing the holistic value of UGI [17], where LCT generally refers to a set of approaches which include the positive and/or negative social and/or environmental and/or economic consequences of a product or process across its lifetime rather than solely assessing and valuing one, or a few, particular services/benefits or impacts or only focusing on the use phase. An LCT approach has the potential to lead to holistic triplebottom-line whole-of-life assessments and could be applicable to a wide range of green infrastructure (GI) types. A body of literature is emerging on the topic comprising methodology development, discussion and case studies of method application. However, there is currently no overview available of these developments.

Rationale
It is worth emphasising that LCT is not the same as life cycle assessment (LCA) (or analysis). The latter, as codified in the ISO 14040 series of standards, was initially developed to compare the impact of manufactured products, and subsequently services, across a range of predominantly environmental impact categories. What LCA and LCT have in common is the understanding that human-created systems-in this case UGI-comprise several life stages, from planning and design through installation, management and maintenance, and finally end-of-service-life disposal (which may include elements of reuse and recycling). The initial motivation for this study was a recognition among the authors that most evaluation of UGI was based on a snapshot view of performance-right here, right now-without integration of the overall effects, both positive and negative, of that UGI over its entire service life. Even 'natural' GI in a city, such as remnant indigenous vegetation, will require at least some management intervention, leading to the existance of both benefits and costs/disbenefits associated with it, and both positive and negative environmental, social and economic effects can be identified when an LCT framework is applied. LCA and the related methods are however, seen as one type/group of methodological basis for the practical application of LCT. The other key methodology group considered in this review is cost-benefit assessments (CBAs and related methods) and considered as part of LCT where they have been applied to assess more than one UGI life cycle stage.
The evidence base on methodological developments and practical applications of LCT methods for UGI assessments-a novel field-is sparse, limited in scope and scattered among diverse disciplines (see for example section 3.3.4, which identifies 19 research fields based on SCOPUS classification) and is lacking an overview of the current state-of-the-art [16]. An interdisciplinary methodological conversation is currently emerging in the literature and would be greatly supported by a mapping of the current evidence landscape and identification of current advances, challenges and gaps.
To the best knowledge of the authors, only four literature reviews had been published on the topic at the time of this review. Belussi and Barozzi [18] in 2015 reviewed literature of life cycle based environmental and economic impact assessments of measures addressing urban heat islands. This included building elements, urban pavements and urban green spaces, and articles on both 'green' and 'grey' measures in the analysis. The authors identified 16 papers that looked at GI from a life cycle perspective. In 2017 Petit-Boix et al [17] identified the key strategic fields of urban sustainability, including 'green spaces and landscape' , and reviewed published LCT-related assessments within each field. The analysis of assessments within 'green spaces and landscapes' included ten papers. Rincón et al [19] and Pérez and Perini [20] provided two review chapters in a book each focussing on LCT assessments of a specific type of UGI-green roofs and green walls respectively. Rincón et al [19] reviewed four articles on green roofs, Pérez and Perini [20] looked at three LCA studies of vertical greening systems.
These reviews have not been systematic and/or have focused either narrowly on a particular type of GI, or widely on broader urban sustainability assessments mixing green and non-GI approaches. Hence, an overarching and systematic overview of the evidence base regarding current applications of a LCT approach for UGI assessments is lacking. This study aims to close this gap and provide a knowledge base for the future development, adaptation and consolidation of LCT methodologies for holistic UGI assessments, and to inform UGI practice more generally, including its design, installation and maintenance.

Objectives
The first objective of this systematic scoping review is to map the current landscape of peer-reviewed literature on the LCT approach for holistic UGI assessments/valuation. The research question posed to achieve this objective is: what is the current landscape of peer-reviewed studies applying LCT in UGI assessments in terms of: (a) number of articles published; (b) publication rate over time, (c) geography and types of author affiliations and collaborations; (d) distribution across fields of research and journals and (e) identification of key seminal and influential studies.
The second objective is to provide an overview of the current use of various LCT methods, coverage of UGI types in assessments, the range of benefits and costs/ disbenefits assessed, as well as the UGI life cycle stages considered.

Methods
Systematic scoping review methods are used to answer broad questions on the landscape of available literature on a specific topic and its characteristics, as well as knowledge emergence and development over time. This scoping review closely follows the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Extension for Scoping Reviews [21] and presents a thorough overview of the peer-reviewed literature on the application of LCT in UGI assessments.

Protocol and registration
A scoping review protocol was prepared and pre-registered on the Open Science Project preregistration depository (accessible here: https://doi. org/10.17605/OSF.IO/U9285) prior to the commencement of the review and was locked for alterations to ensure transparency, accountability, replicability and robust review planning. It served as the pre-agreed detailed methodology and a guide for the systematic scoping process.
Three methodological deviations from the protocol are to be noted: (1) the protocol did not include the full range of search keywords and search strings denoting the 'urban agriculture' UGI type, this was however deemed to be an important gap and urban agriculture related UGI assessments were included in the study at a later stage following the same logic as all other keywords 1 ; (2) the additional reference mining among the final batch of articles was not carried out as the final pool of papers was deemed sufficiently 1 The additional keywords included are: 'urban farming' , 'urban farm' and 'urban agriculture' . The generic search string for these keywords followed the same logic as all other keywords: ('urban farming' OR 'urban farm' OR 'urban agriculture') AND ('life cycle' OR 'whole-of-life' OR 'whole life' OR 'cost-benefit' OR 'cost benefit' OR 'CBA' OR 'LCA' OR 'LCC' OR 'SLCA' OR 'LCSA') AND (assess * OR valu * OR evaluat * OR apprais * OR costing OR analysis OR rating) AND (urban OR city OR cities OR metropol * OR municipal * OR built OR building * OR neighbo?rhood) . large, (3) the full-text screening and data extraction was carried out by one expert per paper (however the preliminary title and abstract screenings were carried out as planned in the protocol-by two independent experts each, with third expert resolving conflicting assessments).

Eligibility criteria
The eligibility criteria for inclusion of articles in the scoping review were defined for both (a) technical eligibility-based on publication characteristics and (b) content eligibility-based on alignment with the research objectives.
The technical eligibility was determined as follows.
Eligible publication types: • peer-reviewed journal articles, • peer-reviewed books and chapters • edited collections of articles with full texts included • peer-reviewed conference proceedings.
Publication characteristics: • Language: English • Publication status: any (e.g. pre-print, article in press, early access, final etc) • Publication years considered: any To find answers to the research questions, the following content eligibility criteria were applied for the selection of the papers:  c. An assessment case of: (i) an actual UGI object (ii) test prototype of an UGI object (iii) theoretical example of UGI object. Figure 1 shows the inclusion decision-tree based on the eligibility criteria.

Information sources
An initial comparison of 12 databases based on the size of the relevant results pool, coverage of the topical keywords, overlaps, usability and focus on peerreviewed literature was carried out and documented in the pre-registered Scoping Protocol. It led to the choice of three scientific databases for the searches: Scopus, Web of Science and ProQuest (see the preregistered Scoping Protocol 4 for the full database analysis).

Search
The search was implemented on 5 April 2019, applying the detailed search strings specified in the Scoping Protocol 5 for each database. The additional searches applying additional keywords relating to 'urban agriculture' were carried out on 20 May, 2022.
The range of search terms chosen for the literature search represented the five core themes, which set the boundaries of the study (the search also included all relevant and synonymous terms): • GI • Assessment • Life cycle approach • Costs and benefits • Urban setting Seventeen general search strings were defined, each constructed around one keyword representing UGI or its types: GI, urban green, green urban area, green assets, green wall(s), green roof(s), garden(s), street greening, green space(s), greenery, urban forest, vegetated, park(s), nature-based measures, ecosystem-based measures, urban ecosystem(s) and urban agriculture. This approach enabled later analyses of keyword popularity. The search strings were adapted to the logic and the specific use of Boolean operators in each of the three databases. The full list of the 51 applied search strings (17 search strings for each database) is available in the Scoping Protocol 6 .
The search was not limited to a specific time range, as the aim was to capture all relevant articles regardless of their publication date to account for the emergence and historical development of concepts and methods and allow a temporal analysis of the scientific literature.
The search was carried out in the following metadata fields: title, abstract, keywords.

Citation management
All citations were imported into bibliographic manager Citavi 6.0 [22] and duplicate citations were removed. The remaining unique items then progressed to the technical eligibility screening (see section 2.6).
For the title and abstract screening, as well as subsequent full-text screening for eligibility the citations were imported into the web-based systematic review software Rayyan [23].

Selection of sources
The PRISMA flow diagram (figure 2) details all steps of record identification, screening and selection and the outcomes of each step [21].
Duplicate removal was followed by metadata technical screening for eligible publication types, eligible publication language (English) and full-text availability.
Then, content eligibility for inclusion in the scoping analysis was screened based on the criteria laid out in section 2.2 in two phases: • Phase 1-Title and abstract screening for inclusion eligibility. • Phase 2-Full-text screening for inclusion eligibility.
Three independent reviewers were involved in the screening. Each item was first independently screened by two reviewers, and in the case of disagreement it was resolved by an independent third reviewer. The final decision regarding inclusion in these cases was taken based on majority vote of all three experts.

Data charting process
All articles found to be relevant after the two screening phases were included in the scoping review for data extraction. During the data extraction 18 further full-texts were found not to deliver sufficient detail or sufficient evidence to comply with the inclusion eligibility criteria. An Excel-based data tabulation template was developed and agreed among the authors alongside an explanatory manual for ensuring uniform interpretation and use of data fields.
The data from each full-text article were extracted by one author along with highlighting the data in the full-text files. The reviewers met regularly to resolve unclear cases or arising questions to ensure consistency between reviewers. Table 1 lists the main categories of data extracted from the final pool of eligible primary studies in relation to the research objectives.

Data items
Due to the broad scope of the review, not all data items collected are reported in this article and could provide a basis for future analyses.

Data summary and synthesis
The data were compiled in a single Excel-based spreadsheet for quality check and validation to correct any gaps or inconsistencies. Information coding was applied to enable further analysis.
Descriptive statistics-measures of distribution (most often frequency distribution), centrality (means and medians and modes), dispersion (ranges) and other (e.g. sums, min and max values)-were calculated and visualised to summarise the data.  Qualitative narrative synthesis guided the reporting of the findings, steered by the research questions as themes.

Search and selection of studies
The initial search resulted in a total of 2239 results, which were eventually screened down to 164 eligible articles included in the analysis. This is illustrated in the PRISMA flow diagram (figure 2). The full list of included studies is provided in supplementary material.

Characteristics of sources of evidence
In summary, 93% of included studies are peerreviewed journal articles, 69% are published in current Scimago Journal Rank Index quartile 1 (Q1) journals and 80% in total are published in Q1 and Q2 journals (figure 3). Thus we concluded that the final selected study pool was of sufficient quality for further analysis. The number of citations per publication range from zero to more than 508 citations, with around one third presenting 50 or more citations (the number of citations is dependent on several factors including the date of publication-more recent articles typically have fewer accrued citations).

Timeline of studies
The first published study found to be applying LCT 7 to UGI assessments dates to 1992 (see figure 4). Until 2003 articles are limited and mostly originate from the same research group within the United States Department of Agriculture (USDA) Forest Service who carried out the early work in this field. However, these early studies have no direct mention of any existing LCA methods (they are picked up by this scoping exercise because the search and eligibility criteria were defined in a way that would capture LCT even if it is not explicitly stated). We name this first early period of development Phase 1: Early Thinkers (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002).
In Phase 2: First Adopters (2003-2009) an increase of studies bearing the characteristics of LCT is observable, averaging at 2.6 articles per year, authored by a more diverse set of authorsincreasingly from academic institutions-and from a growing number of countries. We also see the first papers explicitly applying LCA and LCC. The topic gains increased traction between 2010 and 2015-Phase 3: First Diffusion-with an average of ten articles per year and accelerated expansion of author diversity, multidisciplinarity, geographic coverage and types of GI being assessed.
The last 5 years of our review period (until 2019) see a further doubling of published article numbers per year, indicating a rapid spread of life-cycling thinking in UGI assessments in diverse research fields. Hence, we name this period Phase 4: Accelerated Spread and consider it currently ongoing.

Author organisations and collaborations
The share of papers authored by one versus several organisations splits at 41% and 59% respectively ( figure 5). Likewise, we observe a relatively equal split between multidisciplinary 8 (53%) and singlediscipline (47%) research. Almost half (47%) of the research is both multidisciplinary and includes multiorganisation collaborations. A more detailed analysis of the types of author organisations reveals that around 66% of papers come solely from academic institutions, 26% are collaborative between academia and practice-oriented entities, while 7% have been published with no involvement of academic institutions, even though our search was carried out exclusively in peer-reviewed research publication databases. In total more than a third of papers have been developed in collaboration with practice and decision-making organisations either in public or private sectors, which is an indication of the applied nature of the subject. There are likewise pronounced trends of increasing collaboration, diversity of organisation types and interdisciplinarity over time. In Phase 1: Early Thinkers all papers are authored by one organisation in one discipline. In Phases 2 and 3 around half the studies are multi-disciplinary multi-organisation projects, rising to 55% with the start of the Accelerated Spread Phase. Although the first Phase started outside the academic setting, the academic research community has been quick to pick up and start leading the study pool from Phase 2 onwards, significantly increasing their share of authorship in each subsequent Phase (up to 70% in Phase 4), at the same time increasing collaboration not only among themselves, but also with a broad range of policy and practice organisations.

Geography of studies
Author organisations from 38 countries are represented in this review. They are predominantly located in Europe (represented in 61% of publications), North America (43%) and Asia (24%). On a country level 9 , the United States is most frequent, with presence in almost 40% of papers in the final pool (figure 6), followed by Spain, Italy, China and Germany. This pattern aligns with the Nature Index top listed countries with the highest research output both in overall terms, and in Earth and Environmental Sciences specifically (1 June 2019-31 May 2020) [5]. We note some differences with the Scimago Country Rank 1996-2019 in the subject area of Environmental Science [6] with a less frequent presence of India, Canada and Australia in our data. We also observe significantly higher weight of contributions from Italy and Spain indicating that researchers in these two countries could have a special interest in the topic.
However, it needs to be noted that this scoping review only included publications in English indexed by three scientific databases, which would likely bias the results towards research originating in predominantly English-speaking countries and those routinely publishing in journals indexed by these databases.
There are also country-based differences to international collaboration evident in the pool of selected articles. Altogether 34% of papers stem from international collaborations. However, the distribution among countries is not proportionate. While the USA contributed the highest number of papers and also the highest absolute number of internationally collaborative articles, the relative share of international papers (co)authored by the US is only 32%.  Europe has already previously been found to be the most internationally collaborative region in research (largely due to EU-wide common multi-annual research programming and funding) [7], which is also reflected in our dataset, with Finland being the only European country not following this pattern in our dataset.
Over time there is an increasing trend of international collaboration in the field. More than 25% of articles in Phase 3 are co-authored by authors based in more than one country (see table 2). Although, with a delay, this is in line with the general increasing trajectory of international collaboration in science [8].

Subject areas
We find that the papers applying LCT to UGI assessments have been published in diverse subject areas and fields (determined by the subject(s) assigned to the journal where the article was published in-as based on SCOPUS classification of subject areas and fields [9]).
Overall, 19 fields in all four subject areas of science as defined by SCOPUS are represented. This is illustrated in figure 7, where each 'bubble' documents the publication of a single paper/article in four subject areas (with the fifth line being those where the subject area is unknown). The 19 fields further divide into 72 sub-fields. While the majority of papers are published within the area of Physical Sciences (72%, predominantly in the fields of Environmental Science and Engineering) a considerable share of articles also occurs in the areas of Social Sciences and Humanities (17%, mostly in the field of General Social Sciences) and Life Sciences (9%, mainly in Agricultural and Biological Sciences journals).
The landscape of the subject areas and fields, which are associated with UGI assessment publications has expanded significantly over time (table 3).
The early Phase 1 papers until 2003 are all published in either the field of Environmental Science, Agricultural and Biological sciences and Earth and Planetary Science (Physical and Life science areas); from then the range of fields expand to Social Sciences and Engineering (figure 7). By the end of Phase 2 (2009) the range of fields also broadens to Medicine, Chemistry and Arts and Humanities (from then on representing all four research areas). In 2013, during Phase 3, further fields-Materials Science, Energy and Economics, as well as Econometrics and Finance-are added. By the end of Phase 3 in 2015 articles published in Planning and Management, Earth and Planetary Sciences, Toxicology, Decision Sciences, Computer Science as well as Business Management and Accounting join the pool. In Phase 4, so far, we are seeing a slightly lower range of fields in relation to the topic, but a higher range of subfields; however, this Phase is considered to be ongoing and therefore this spread is not complete.

Landscape of journals
A diversity of subject areas and fields implies a diversity of journals represented in the final pool of included studies. Indeed, the articles included in the review have appeared in 83 different journals (or proceedings or books) averaging at only two (median 1) articles per journal. The journal publishing on the topic most frequently-Journal of Cleaner Production-hosts only 10% of all papers. The top six journals, all of which are Q1 journals, represent one third of all papers.
67% of the journals (and books or proceedings) host only one relevant article making it challenging for researchers to track and follow studies on the subject.
Likewise, there is no significant clustering occurring between the UGI types assessed and the journals. Although there is a slight tendency for swale  and bioretention cell assessments to be published in hydrology related journals, green roof and green wall assessments to appear in building related journals, tree (single or multiple) assessments to favour urban forestry and arboriculture journals and urban forestry related articles to favour general environmental science journals, most UGI type assessments are spread between a wide variety of journals.
Only two of the top six journals by number of published articles (table 4) are also in top six by average citations per article. However, when all Q1 journals are considered regardless of their popularity in the review pool, they do account for the highest number of average citations per paper (52 citations) with Q3 journal articles being the second most cited on average (35 citations).

Seminal and influential studies
The earliest articles we find, which apply the principles of LCT for UGI assessments, are credited to the USDA Forest Service research group led by Gregory McPherson and colleagues. They authored the first paper in our search results (in 1992)-on cost benefit accounting of urban green space [24] and have led or participated in five other published research articles during the Early Thinkers and First Adopters Phases (between 1992 and 2009) as well as three further papers in the First Diffusion Phase (2010-2015). Their focus has predominantly been on the assessments of urban trees. McPherson is also the overall most prolific (co)author in the selected study pool with nine papers in total, half of those in the very early phases. Three of these initial USDA publications have also been cited more than 100 times (although they are not the most cited in the pool), and therefore could potentially be considered 'seminal' [24][25][26].
In Phase 2 the number of articles on the topic rapidly expand leading to the emergence of several influential papers-six from this phase have received more than 250 citations, all on green roofs [27][28][29][30][31][32]. Three studies stand out in particular. Carter and Keeler 2008 [32] (second most cited paper in the pool) did a life cycle cost-benefit analysis of extensive green roofs based on data from an experimental green roof  [30] (fourth most cited in the pool) assessed the life cycle benefits-primarily energy saving-from adding a green roof onto an eight-storey residential building in Madrid.
Around half of the articles in our selection with more than 100 citations were published during Phase 3. The most cited paper in the pool, Susca et al 2011 [33] has been cited more than 500 times for their work on comparing the life cycle impact on climate (CO 2 equiv.) of black, white and green roofs and their ability to alleviate the urban heat island effect in New York City. Another notable mention from this phase is Bianchini and Hewage 2012 [34] who pose the question 'How green are the green roofs?' and focus on the life cycle environmental impacts of green roof materials.
The papers with more than 100 citations exhibit common characteristics. Altogether they assess 13 types of UGI, however, there is a clear focus on two main classes-green roofs (extensive and intensive) (17 assessments), urban farms (12 assessments) and urban trees (multiple dispersed, lined or single) (seven assessments). The popularity of green roofs aligns with their overall dominance as the most assessed UGI type in the whole pool-discussed later. The most cited papers are predominantly written by authors affiliated solely with academic institutions (62%) in only one country (72%)-mostly the US-and tend to represent only one discipline (55%). Interestingly, the more multi-disciplinary, international and academic-public-private collaborations have, on average, received fewer citations.

Study types
When analysing the study types in the final set of articles we collected data on (a) whether the study is assessing a theoretical UGI case or an existing UGI object (full scale or test plot) or alternatively deals with assessment methodology development without application to a specific case; and (b) whether it is a comparative assessment and, if so -what type of comparison is being made.
Only 20% of the studies in the pool are assessments of existing real life UGI; the majority (54%) look at theoretical cases (see table 5). Note: several papers assess both real and theoretical UGI, therefore the total sum in the table is higher than the number of articles reviewed.
A majority of the reviewed papers (69%) are comparative studies (table 5). Most often UGI is compared with alternative 'grey' infrastructure (e.g. green roof vs. conventional roof) (table 5). However, almost one fourth of the articles have focused on comparing different configurations of the same or similar UGI type (e.g. extensive green roof vs. intensive green roof or one species of a tree vs. another species or different urban farm set-ups). Comparisons between dissimilar UGI types (e.g. green roof vs. street trees for air cooling) are less frequent. We observe a range of other comparison types as well: a) comparison of the same UGI in different locations, b) comparison between UGI and UGI in a combination with other green, blue (water bodies) or grey measures, c) comparison of UGI with other solutions (e.g. smart city solutions) that do not fulfil a similar purpose. There is a clear trend of an increasing number of comparative studies across the timeline of Phases. In Phase 1 all studies are non-comparative, with the share of comparative studies steadily increasing over time, in the last 10 years 60% to 90% share of cases being comparative.

UGI types assessed
Along with an increasing number of articles being published, the diversity of UGI types assessed has also increased with time. In the Early Thinkers Phase just one type of UGI was being assessed (urban trees). In Phase 2 we already see up to five types of UGI being assessed in a given year, increasing further to a maximum of nine UGI types per year in Phase 3 and reaching between nine and 14 types of UGI assessed in Phase 4 years (with the exception of 2019, which was the year of the review, and therefore incomplete) (figure 8-the data in figure 8 are at the main UGI type level e.g. all types of green roofs are counted as one type-'green roof ').
Nevertheless, the LCT assessment landscape is heavily dominated by one UGI type-green roofs, with 40% of studies 10 assessing intensive or extensive green roofs (figure 9-the data in figure 9 is at sub-type level, e.g. 'green roofs' are broken down into 'extensive' and 'intensive' sub-types). The second most assessed UGI type is urban farms (subject of assessment in 15% of studies in the pool), and thirdurban trees (included in 10% of all assessments).
The strong focus on green roofs, which starts from 2010, but has rapidly increased especially since 2015, and the assessments of urban farms rapidly rising from 2014 are the two key drivers of the increasing total number of LCT assessments of UGI. While green 10 There may be one or several UGI types assessed per paper. roofs reach up to 18 inclusions in assessments per annum and urban farms up to nine, no other UGI type has exceeded more than five LCT assessments in a given year. When summed together with green walls, which also have received a fair amount of attention, it emerges that the overall focus of assessments has been on UGI types of smaller scale, with clear physical delineation and integrated with and directly dependent on built infrastructure (buildings or other structures).
Urban trees are the third most assessed UGI type in LCT studies. Most studies deal with UGI consisting of many dispersed trees such as all urban trees in a city or a tree planting programme (11 cases) and five assessments focus on single trees; a smaller number of studies analyse trees lining streets. More complex systems that also include trees as key componentssuch as parks or forests 11 are assessed less frequently (only three cases).
A fourth group of UGI types that stands out are the various water management measures. This group of measures is highly diverse, with raingardens and bioretention cells receiving the most attention (18 publications), followed by swales and trenches (nine publications), constructed wetlands and permeable pavements (nine cases each). Water management is clearly one of the key current intended purposes of UGI, requiring an evidence base for decisionmaking.
The general trend is for more natural, larger scale and more complex systems to be assessed far less frequently, thus decorative gardens, parks, forests and others are currently significantly lagging, meaning that there is overall less understanding of their value to urban areas in the research landscape.

Methods used
The application of the 'life cycle thinking' criterion in our search and screening approach has been defined broadly as 'two or more life cycle stages included in the assessment' . This allows for a broad overview of the different assessment methods of UGI. As a result, in our final pool of eligible studies it is the LCT method group (LCA, LCC and LCSA) and cost-benefit (CBA) method group (CBA and cost effectiveness assessment) that dominate the methodological approaches. The method was recorded as explicitly stated by the authors of the studies-see figure 10.
CBA methods focus on the comparative balance between costs and benefits of an UGI object expressed (most often) in monetary terms and (in most cases) discounted to a net present value. LCT methods focus on the environmental (and in some cases social) impacts of UGI projects.
When applying LCT methods-due to their primary focus on impacts-the 'benefits' are relative and implicit as comparatively lower impacts. They are measured via quantitative indicators that may or may not be expressed in monetised values. Seven per cent of studies apply both CBA and LCA methods for their assessment purposes, typically first determining the environmental impacts via LCA and then integrating those in a monetary CBA. Five cases assess costs or benefits, but not both (applying LCT, CBA or other method).
The first paper we find explicitly applying an LCT-related method to UGI was published in 2006-14 years after the first published article in the pool. Rapid uptake and use of the LCT group of methods for UGI assessments starts from 2010 onwards.
Upon more detailed analysis of the studies applying LCT methods, it becomes evident that very few can be considered 'full LCA' , as there are considerable gaps in the coverage of life cycle stages, benefits and disbenefits assessed, which is discussed later.

Life cycle stages included
For the purposes of this review we used the division of UGI life cycle into seven stages based on the ISO 14040:2006 standard [35] with variations as described: (1) raw material extraction and upstream production (including transport) stage, (2) component manufacture (including transport) stage, (3) material or component purchase stage (this stage was used for those studies which did not detail the preceding stages, but included benefits or more often costs of purchasing the components), (4) design stage (often not separated out in the ISO framework, but explicitly addressed as a separate stage in some studies), (5) installation and planting stage, (6) use/operation stage, which includes maintenance, (7) end-of life stage-disposal, waste treatment, recycling (including transport).
The life cycle stages assessed for benefits and disbenefits were different in the majority of reviewed studies, with some studies only assessing the benefit side or only the cost side ( figure 11 illustrates the life cycle stages' coverage in benefit versus disbenefit assessments). Those studies where benefits were framed as comparatively lower impacts/disbenefits (e.g. comparatively lower energy use), the life cycle stages match on both sides of the equation, as essentially the analysis is assessing the disbenefits and then comparing to identify the better performing solution (the one having less negative impacts, thus comparatively more beneficial). This is often the case of the comparative studies, which explore whether UGI has a less detrimental impact over traditional, grey infrastructure. It needs to be noted that these types of assessments essentially focus on 'doing less bad' rather than 'doing more good' and may leave out other benefits.
On average the reviewed studies include 1.9 life cycle stages for benefits and 3.3 life cycle stages for costs/disbenefits. These averages are swayed by a few studies that assess a high number of stages, as the medians are 1 for benefits and 3 for disbenefits. The inclusion of one to three stages means that many life cycle stages are routinely not being assessed (usually the stages up-and down-stream from installation and use). Therefore, only very few studies in the pool can be considered comprehensive LCAs.
As can be expected, there are notable differences between the life cycle stages covered by the different methodologies. The LCT method group typically focuses on the impacts or disbenefits over several life cycle stages. The CBA method group focuses on both costs and benefits, and primarily looks at the monetary cost of installation and maintenance stages with benefits mostly limited to the operational stage.
We also observe differences in the coverage of life cycle stages per UGI type. The assessments of the most frequently analysed UGI type (green roofs) on average do not include the highest number of life cycle stages. It is rather in the assessments of the less frequently analysed and more complex UGIs and water management related UGIs where we see a higher number of life cycle stages included-trenches, ornamental plantings, parks, raingardens, swales and bioretention cells (on average 2-5 life cycle stages assessed for benefits and 4-6 stages assessed for disbenefits and costs). An exception is urban farms, which are both a frequently assessed UGI type and also have been assessed for a comparatively high number of LCA stages-2.7 stages for benefits and 3.5 stages for disbenefits While initially there was a notable increase of the number of LCA stages being assessed between the Early Thinkers phase and First Adopters phase, thereafter we observe only a slightly increasing trend. Typically, the coverage fluctuates across the years, but has not increased notably since the second phase. The increase is slightly higher for disbenefit assessments.
It is known from previous work [16] that the authors of UGI assessments frequently identify impacts throughout all lifecycle stages as nonnegligible and/or state that omission of life cycle stages is a limitation likely to influence the results.
On the other hand, some argue that certain stages represent negligible costs or benefits and therefore their exclusions do not represent a significant shortcoming.

UGI benefits assessed
The authors' own expert knowledge and a literature review was used to identify a comprehensive list of benefits of UGI (see figure 12 and supplementary material) to determine the frequency of their assessment. Where additional benefit types were identified in the reviewed studies, they were added to the list. There is inevitably an element of value judgement in the categorisation of benefits and disbenefits. Key considerations here are transparency-how benefits/disbenefits are described, and consistencythe same terminology is applied across the range of papers being analysed. The 164 papers in the review as a group assess 823 instances of UGI benefits, averaging at approximately five benefits per study (median-four), with the maximum number of assessed benefits being 15-16 [34,[36][37][38]. Almost two thirds of the studies assess one to five benefits, one third six to ten benefits and only 8% include more than ten benefits in their scope ( figure 13). Five percent of the studies assess only disbenefits.
The top two most assessed UGI benefits by a considerable margin are 'energy savings/improvements of energy efficiency' and 'greenhouse gas (GHG) emissions reductions' (figure 12). These two benefits are often assessed in combination because GHG emissions reduction is commonly derived from the building-related energy savings. We observe that predominantly these are energy savings in the operational life cycle stage of the UGI, and not savings across all life cycle phases. Similarly, another closely related benefit is the 'energy-related cost saving' , which occupies fifth place in the most assessed benefits.
The first most frequent provisioning service ('food production') is assessed in 20 cases and first most popular social benefit ('aesthetics')-in 16 cases; another popular social benefit-'direct benefits to human health (physical)'-has been analysed in 12 cases (note however that when 'aesthetics' is grouped together with 'local distinctiveness' , this benefit groups receives less attention than the combined human health and recreation benefit group) 'Other' additional benefits identified include dust interception, improved access to subsidies, household savings from growing own food, and the social benefits of the UGI.
We grouped the identified UGI benefits into the triple-bottom-line categories: (1) environmental pillar benefits (environmental quality for both human and ecosystem benefit, climate change mitigation, climate regulation for indoor and outdoor comfort, water flow management, resource preservation/efficiency), (2) social pillar benefits (human health and  recreation, job creation and education, aesthetic and local distinctiveness, social cohesion, crime reduction and equality enhancements) and (3) economic pillar benefits (income from UGI use, taxation or carbon/ biodiversity credits, cost and tax/fee savings, increased property value, provision of food, materials and energy and reduced load on grey infrastructure). The full list and grouping of the assessed benefits is available in the supplementary material. When looking at the aggregated benefit type groups and pillars, the unequal attention given to the different aspects of the triple-bottom-line becomes apparent ( figure 14).
Environmental benefits are the most assessed benefits of the three pillars of sustainability (included in 87% of studies- figure 14)-largely driven by resource efficiency benefits (predominantly energy efficiency-74 assessments), climate change mitigation (mostly GHG emissions reduction-70 assessments See figure 15), and environmental quality for ecosystem benefit (mainly ozone depletion-28 assessments) or human benefit (mostly outdoor air quality improvement-47 assessments).
Economic pillar benefits are included in 61% of reviewed papers ( figure 14). 'Cost and tax/fee savings' is the most popular economic benefit group More specifically, the most assessed cost saving benefits in the cost and tax savings group are 'energy cost savings' (42 assessments) and 'reduced costs for related grey infrastructure' (32 assessments) ( figure 15).
However, only around 20% of papers include the social pillar ( figure 14), which represents 6% of the total assessed benefit count (n = 46 out of 823 figure 15). Human health and recreation is the most frequently analysed social benefit group, predominantly focusing on physical health (12 assessments). There were no studies in the review pool which only assessed social benefits and there were likewise no studies assessing the combination of social and economic benefits. There is a small percentage-5%which assess the combination of social and environmental benefits. It appears that social benefits are mostly included in studies which strive towards triple-bottom-line assessments, as the social aspects are most often included in combination with both environmental and economic benefits (14% of studies) ( figure 14).
Seven of the 65 identified UGI benefits have not been assessed by any of the studies included in the review-these are predominantly social benefits (reduction of social inequality, crime prevention/reduction, local distinctiveness), provisioning services (medicinal material and fibre production), and more advanced financial benefits (UGI asset value, income from the sales of biodiversity credits or similar).
The average number of benefits assessed per paper fluctuates year on year with a slight downward trend over time; some of the studies assessing higher than average number of benefits were published in the early phases. While environmental benefits have been assessed throughout the whole period under review, the first assessments including economic benefits appear towards the end of Phase 2 (as energy cost, infrastructure cost savings and income from carbon There are notable differences in the number of benefits assessed by studies that apply LCT or CBA methods. Life cycle-thinking assessment methods primarily look at impacts (disbenefits) rather than benefits-the majority of LCT studies assess only one to three benefits (45%) and a considerable share (8%) focus solely on disbenefits. From the perspective of the benefit range typically assessed per UGI type, there is a pattern for assessments of water management related UGI (especially pervious/ permeable pavements, swales, trenches, raingardens and bioretention cells) to include a higher number of benefits compared to the other types of UGI-often around 5-7.5 benefits assessment ( figure 16). Green roof assessments likewise typically include a comparatively high number of benefits-6-6.5 on average. One type of tree-related UGI-linearly arranged street trees are also assessed for six benefits on average. On the other hand, dispersed trees, all types of green walls and urban farms fall in the middle with 4.5-5 benefits per assessment. Constructed wetlands (4.2), lawns (3.5), single trees (2.7) and decorative gardens (2) are the least comprehensively assessed UGI types in terms of benefit coverage ( figure 16).

UGI costs/disbenefits assessed
A range of 36 disbenefit/cost types were identified in literature and the authors' knowledge of the subject, with several types additionally identified in the reviewed papers and included in the final list (see additional material and figure 18).
The overall average number of disbenefits assessed per study is four. While it fluctuates between two and six in a given year, overall, there is almost no notable upward or downward trend over time with only very slight tendency towards decreased number of disbenefits being assessed.
The maximum number of disbenefits assessed is 13 and only one study assesses no disbenefits. More than half of the studies (55%) include one to three disbenefits ( figure 17). Out of those the majority (48 papers) include two cost/ disbenefit types ( figure 17). Almost one third assess four to six disbenefits. Only one article includes the maximum observed range-13 disbenefits.
The most assessed cost/disbenefit types by a large margin are the monetary costs for implementation (95 cases) and maintenance (93 cases) of the UGI ( figure 18). This illustrates the popular approach of contrasting (comparative) benefits of UGI against monetary costs, which we observe not only in the CBA method group studies, but also in the LCT method group cases. The third most popular disbenefit type is GHG emissions (86 cases), followed by water (47) and air (39) quality issues, and energy demand (38). Impacts causing a reduction of quality and quantity of habitats and biodiversity, comparatively lower effectiveness and increased costs for the related grey infrastructure, impacts on air temperature and aesthetics are all rarely measured. Nine of the potential identified disbenefits were not assessed in any of the reviewed studies.
Similarly to the benefits, we grouped the costs/disbenefits into the triple-bottom-line categories: (1) environmental pillar disbenefits (environmental quality, resource depletion, climate change), (2) social pillar disbenefits (impacts on aesthetic and local distinctiveness, human health and recreation, social cohesion, crime and equality) and (3) economic pillar disbenefits (decreased income, decreased property value, increased monetary costs, damage to infrastructure and property and lower performance effectiveness groups). The full list and grouping of the assessed benefits is available in the supplementary material.
While several of the economic costs and disbenefits are the most assessed disbenefit types, overall, the economic disbenefit pillar is being assessed as often as the environmental one (in 60% of papers and 63% papers respectively - figure 19). Direct monetary costs have been assessed in 220 instances (out of 649 total assessment instances across all disbenefit types- figure 20), heavily dominated by implementation and maintenance costs/cost increase (assessed in 88 and 86 cases respectively).
As with benefits, social pillar disbenefits are the least assessed. Where they are included, it is always in combination with the other pillars. Social aspects are only included in those studies that also include environmental disbenefit analysis, meaning that no social-economic disbenefit analyses were found. Altogether social disbenefits are assessed in 18% of papers ( figure 19) with the most often assessed social disbenefit group being 'human health and recreation' (27 instances- figure 20). Contrary to the benefit assessments, the studies which employ LCT methods include a higher number of disbenefits-4.7 on average-than studies applying CBA approaches-three disbenefits assessed on average. Assessments which combine both method types average 4.4 included disbenefits. Most LCT-based studies (38%) assess one to three disbenefits. A further 27% assess four to six disbenefits and 21% include seven to nine disbenefits, with the remaining 14% of papers assessing 10 or more disbenefits. Those assessments employing CBA methods tend to include a low disbenefit count more often-45% of CBA-based studies assess one to three disbenefits only. These differences may be explained by the focus of the respective methods-LCT studies aim to assess a range of diverse environmental impacts/disbenefits, while CBA studies primarily focus on assessing the main monetary costs, which are less diverse.
On average, water management related UGI types see the highest number of costs/disbenefits being assessed (figure 21). Single and multiple trees and parks, which include a comparatively high number of benefits (figure 16), seen a lower number of disbenefits measured ( figure 21). The number of costs/disbenefits assessed for green roofs fall in the middle with around four disbenefits per assessment on average (figure 21), which is slightly lower than the benefit count for these assessments (figure 16) Urban farms likewise are being assessed for around four disbenefits on average. Green walls, trees, pervious pavements, decorative gardens/yards and lawns are assessed less comprehensively on the cost/disbenefit side with an average of fewer than three disbenefits included (figure 21).

Publication landscape and knowledge development
We employed broad inclusion criteria to capture studies which have explicitly or implicitly demonstrated elements of life cycle thinking in UGI assessments without publication time limitation.
We observe trends of increasing numbers of articles being published, increasing cross-disciplinarity, growing frequency of cross-sectoral and international collaborations on the topic, expanding scope in terms of UGI types covered, benefits and disbenefits assessed and increasing methodological diversity of studies over time.
Based on the dominating characteristics, we can identify four phases of knowledge development on applying LCT in UGI assessments so far. Phase 1 Early Thinkers spans the first decade (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002), and is characterised by a sparse number of articles, which only implicitly apply some elements of LCT. All studies in this phase originate from one research group in the US public sector, only assess urban trees and do not explicitly reference LCT methods. These are considered key seminal studies, and the work initiating this phase-on tree valuation by the research group at US Forestry Service-is ongoing to this day. In Phase 2 First Adopters (2003-2009), authors from five countries and increasingly from academic institutions join the author pool publishing up to four journal papers per year in four different research subject areas. It is in this phase we see first papers explicitly applying specific LCT methods: LCA and LCC. The studies continue to predominantly assess urban trees, but also start expanding the focus towards green roofs and water management related UGI. For the first time they also include social benefits (aesthetics and physical health) in the analyses.
In Phase 3 First Diffusion (2010-2015) we see an accelerated expansion of the number of articles (ten articles per year on average), author diversity, multidisciplinarity, geographic coverage and types of GI being assessed. Green roof assessments begin to dominate the assessed UGI landscape, however the assessments now cover a considerably broader range of UGI types, also including green walls, urban farms and several additional water management related UGI (constructed wetlands, bioswales, raingardens, bioretention cells). Phase 4 Accelerated Spread started in 2016 and is considered ongoing. During this Phase we see a further doubling of published paper numbers on the topic with 20-30 studies published per year by authors from 35 countries. We note a proportionally higher number of contributions from Italy and Spain (in comparison with Nature Index top listed countries with the highest research output in Earth and Environmental Sciences), which are warm Mediterranean climate countries experiencing heat and water related impacts of climate change and both are located in Europe where there has been substantial research funding available to support research on GI/naturebased solutions.
We observe a very high diversity of subject areas and journals in which UGI assessments are being published and can conclude that there is no clear core group of journals attracting the highest number and/or most cited articles dealing with LCT related assessments of UGI. This is likely due to the high variety of UGI types, the interdisciplinary nature of LCT assessments and high diversity of aims and purposes of the assessments. This would lead to difficulties for those in the field to have a full overview of the generated knowledge. The followers of one subject area and any one of the most popular journals and even those who read all of the most popular journals (which represent different areas of science), might not be aware of the majority of articles on the topic appearing elsewhere, which confirms the value of a broad scoping review across all areas and fields of research.
We note that most of the highly cited papers have been published in Phase 2 and Phase 3 and predominantly focus on LCA of green roofs and urban farms. While the number of citations serves as a reasonable proxy, it does not capture whether the subsequent citations have occurred in papers on the same topic and therefore influential for furthering LCT approaches to UGI assessments. It also needs to be noted that more recent papers may have not yet achieved significant citations, which accumulate over time. A deeper understanding of how significant these highly cited articles have been would require a more in-depth analysis of the citation networks.

Methodologies employed in UGI assessments
As to the methodological strategies employed by the studies, there is a clear prevalence of theoretical UGI cases (54%) over existing UGI object assessments (20%). The theoretical UGI assessments are likely to be of use for both theoretical methodology development and testing of approaches and tools as well as for future-oriented decision-making support in UGI policy development, programming and design.
The real-life UGI assessments provide an insight in the functioning and cost-benefit efficiency of existing UGI and for ex-post evaluations of UGI policies or programmes.
We also observe a large (84% of all studies) and increasing share of comparative studies, with only a few studies in the later Phases being noncomparative, potentially indicating a shift in the role of the LCT assessments to support urban planning or property development and management choices between grey and green or different types and configurations of GI or combinations of both. The comparative studies most often compared UGI with 'grey' infrastructure. This reflects one of the main purposes and aims of holistic LCT method assessments-to enable fair comparisons between dissimilar products/objects/ solutions. One third of the studies have also compared different configurations of the same or similar UGI type. These assessments can aid decisionmaking on the layout, composition, technologies and materials for UGI design and development. Less frequent is an expanding range of more complex comparison types which may be indicative of the growing interest to include UGI options in broader urban design and management decision-making settings.
The two main assessment method groups employed are the LCT method group and CBA method group. CBA was more popular in the early UGI assessments, presumably because it has a longer history as a commonly used project appraisal method and is more universally applicable. Conversely, in order to apply LCT to UGI, certain adaptations are necessary as LCT methods have primarily been designed for products and services, not natural capital. LCT method application to UGI raises specific challenges around defining system boundaries, dealing with perpetually changing systems and wide ranges of benefits and impacts/disbenefits [16]. Thus, most of the early LCT assessments of UGI also serve as methodology development and refinement efforts. In part it is also true for CBA method applications, as several of the challenges are shared across methods (e.g. determining the duration of 'life' of UGI, choice of discount rate, accounting for the full range of benefits, and others). Although CBA methods have been dominating in the initial Phases of knowledge development with first explicit application of LCT methods in the pool found only in 2003, overall we find more studies applying LCT methods and 7% of assessments combining both method types.

UGI assessment scope
From our analysis it emerges that assessments have primarily been done on UGI types of smaller scale and lower biological complexity, those with clear physical delineation and those integrated with, and directly dependent on, buildings or other structures. All these characteristics apply to the most frequently assessed UGI type-green roofs. The second most assessed UGI type-urban agriculture-either matches this pattern (in the case of urban rooftop farms, which share the characteristics of green roofs) or may represent exceptions as larger scale farms, less dependent or urban built infrastructure are also being evaluated (mostly peri-urban). Urban trees, the third most frequently assessed UGI type, is again a smaller scale and lower complexity UGI. The general trend is for more 'natural' (less modified), larger scale and more complex UGI systems to be assessed much less frequently, Figure 21. Average number of costs/disbenefits assessed per GI type. * These UGI types have only been assessed in one publication each, therefore the average is not representative meaning that there is overall less understanding of their value to urban areas.
Most of the reviewed studies include one to three life cycle stages in the assessments, out of seven stages as defined in the ISO 14040:2006 standard [20] for LCA assessments. Several life cycle stages are routinely being omitted from assessment-most commonly the beginning of life cycle and end-of life stages. Therefore, we conclude that while LCT is applied, very few studies in the pool can be considered full LCAs. Authors often cite the lack of data and no established industry practices as the main reason for excluding certain GI life cycle phases from studies [30,[39][40][41]. Furthermore, authors of UGI assessments frequently note non-negligible life cycle impacts throughout all lifecycle stages and/or state that the omission of life cycle stages is a limitation, which is likely to influence the results [39,42], while some have argued that certain stages represent negligible costs or benefits and therefore their exclusion does not result in a significant shortcoming [30,43].
More than half of the reviewed studies assess one to five UGI benefits, but only 8% include more than ten benefits and 5.5% do not assess benefits at all. The limited scope of the majority of current studies is likely to lead to systemic gaps in the valuation of UGI. The benefit range to be included in assessments, however, needs to be seen in the context of the purposes of the assessments and the intended use of outputs, as well as the trade-off between available resources for the assessment and its thoroughness.
We see a similar pattern on the disbenefit side with half of the studies assessing one to three UGI disbenefits and around a third assessing four to six disbenefits. Therefore, the disbenefit side is likewise not accounted for in its entirety in current UGI assessments. In addition, there is no notable change in the number of benefits and disbenefits assessed over time (especially post initial knowledge development phases) which means there is no discernible development towards more comprehensive assessments which could account for the full range of benefits and disbenefits of UGI.
Energy savings and GHG emissions reductions are the most frequently assessed UGI benefits. Considering that the most assessed UGI type is green roofs, this result is not surprising, as one of the main potential green roof (and green wall) benefits is the added insulation and cooling effect that may lead to better energy performance of buildings. Reducing cooling energy use is also typically seen as a key benefit of street trees-the third most often assessed UGI type. The current assessments may not fully accurately represent the overall energy use benefits and disbenefits of UGI, because energy savings are predominantly assessed in the operational life cycle stage, and in only rare cases represent full life cycle energy savings.
Generally, climate change adaptation and mitigation as well as human health benefits appear to be driving a large share of UGI assessments with outdoor air quality improvement for the benefit of human health, water retention capacity increase, and carbon sequestration all being further popular assessed benefits. In addition, outdoor and indoor temperature regulation, albeit somewhat less frequent, reflect an interest in UGI as measures for alleviating urban overheating.
The reduced pressures and demands on grey infrastructure and reduced overall cost savings for grey infrastructure form another often assessed benefit cluster. Understanding the value of UGI as an integral part of urban infrastructure is an important aspect of these assessments.
Water-management related UGI are the focus of around a third of the studies and are most often assessed for their ability to improve water quality, reduce water use and cost and improve water infiltration. These are also recognised as co-benefits of other UGI, such as green roofs.
Economic benefits mostly focus on maintenance and implementation cost savings and increased property values. The least assessed are the social benefits. Aesthetic considerations and direct benefits to human health (physical) receive some attention, but there is very little consideration of other social aspects.
The low share of papers including social pillar benefits and disbenefits reflects the overall slower pace of developments in the area of S-LCA, LCT methods having had traditionally been focussed on environmental impacts. This, however, represents a serious gap in the understanding of the full life cycle value of UGI. Accounting for social aspects is crucial in densely populated urban areas with high level of human-nature interactions, which not only increase the level of the social benefits provided by UGI, but also may exacerbate the disbenefits. It is also of high importance in triple bottom line approaches, where all three pillars of sustainability should receive equal attention.
The assessment characteristics such as the range of benefits and disbenefits included, and the life cycle stages assessed, differ considerably between the different UGI types. Figure 22 illustrates the overall landscape of benefits and disbenefits being assessed in LCT studies for each UGI type. Pervious/permeable pavement assessments show a stark difference between high number of benefits and a low number of disbenefits assessed. Trees (single and multiple trees, including parks) also tend to include higher number of benefits but lower range of disbenefits. Green roof and urban farm assessments tend to be in the average range for both the number of benefits and disbenefits assessed. The differing ranges of benefits and disbenefits assessed per UGI type are indicative of a range of influencing factors including (a) potentially the perceived and recognised range of benefits the respective UGI type provides, but also (b) the specific benefits (or reduction of disbenefits) the UGI might be designed/optimised for or (c) what the key research or decision-making interest revolves around. It needs to be noted, however, that none of the assessments include the full range of benefits the respective UGI may provide, in most cases only covering a fraction of them. The disbenefits are likewise underrepresented, however to a lesser extent. This situation likely leads to UGI being systematically undervalued, and some types of it more than others.

Strengths, limitations and gaps of the scoping review
The key strength of this scoping review is its broad, inclusive approach with the publication date limited only by what is available digitally. This allowed the identification of a large pool of relevant studies across scientific subject areas since the emergence of LCT approaches for UGI assessment, which (as noted by the numbers of papers) emerged with the 'early thinkers' in the 1990s. This is the largest systematic overview of literature on the topic known to date. This scoping review adhered to rigorous and transparent systematic scoping methods throughout and was guided by a pre-registered protocol, which set out the planned study approach with only limited subsequent deviations. Appropriate data management protocols (including regular backups) were observed and the use of systematic review software Rayyan [23] and bibliographic management software Citavi [22] ensured that all citations and articles were fully accounted for during the process. The following main limitations and potential biases are identified for this systematic scoping alongside the measures that were taken to reduce the effect of these limitations and biases on the results. Risk of missing relevant studies: a limited number of databases have been chosen for carrying out the literature search.
To mitigate this risk, detailed database analysis was carried out prior to commencing the scoping review to identify databases with largest potentially relevant and unique contributions (see the pre-registered Scoping Review Protocol accessible here: https://doi. org/10.17605/OSF. IO/U9285).
Exclusion of grey literature sources, which may have added to the pool of knowledge: grey literature has been excluded due to quality considerationslack of peer-review in grey literature publications, and also due to resource limitations of this study; and represents a limitation.
English language focus: only English language publications indexed by predominantly English-language databases where included, which might lead to relevant studies in other languages being omitted. A test search without language limitation revealed only a few papers, some of which were also found in English versions. Non-English articles outside of the chosen scientific databases were not analysed.
Potential errors during the first eligibility screening: reviewers only read the title and abstract in the first screening step, which may not represent the article content fully and therefore lead to the exclusion of some relevant studies.
Full critical appraisal of primary studies was not carried out, even though the extracted data and their analysis included several elements of the typical critical appraisal methodologies for LCA studies. Nevertheless, due to the scoping design, not all reviewed articles followed the methodological design of an LCA study, therefore an overarching uniform quality appraisal was considered unfeasible.
The identification of key seminal studies followed a simplified approach and did not include bibliometric analysis to identify mutual citations within the pool, which is recommended to be addressed in further work.
Restriction to digital evidence: it is possible that earlier papers may have been missed which were not available digitally. Identification of such material would require a paper-based catalogue search.

Recommended further research
The systematic scoping approach followed here has been highly valuable for providing a thorough mapping of the peer-reviewed literature on the emergence and development of LCT in UGI assessments. To further enhance the 'scoping' nature of this review, we propose two further research directions: (1) in this research affiliation has been used a proxy for distinguishing single or multi-discipline studies-future research that examines a higher resolution assessment of disciplines would also be of value to capture perspectives lacking in the literature; (2) better understanding of the knowledge networks and the role and weight of seminal and influential studies may be achieved by applying bibliometric analysis to the pool of identified papers.
Further work beyond scoping is to focus on the content analysis of the identified studies to gain a deeper understanding on (a) the specifics of LCT methodology applications and the implications of methodological choices made, (b) the outcomes of the assessments for the different UGI types and contexts, and (c) the role and added value of incorporating LCT in UGI (and other natural capital/ naturebased solutions) assessments. The collected dataset likewise allows for a more in-depth analysis of the LCT application to individual UGI types to date as well as cross-cutting analysis on the commonalities and disparities between assessments of different UGI types, among other-excluded from this publication due to space limitations.
Finally, the ultimate purpose of this first step scoping LCT application to UGI assessments to date is to support future methodological developments and LCT approach adaptation for UGI (and other green infrastructure, natural capital or nature-based solutions) assessments enabling holistic full-life-cycle assessments and comparisons in support of real-life decision-making.

Data availability statements
The data that support the findings of this study are available upon reasonable request from the authors. Additional supporting information file 'supplementary material' referred to in the article may be found in the online version of this article at the publisher's web site.